The present invention relates generally to methods and apparatus for performing flow cytometry, and more particularly, it is directed to such methods and apparatus for conducting real-time in vivo quantification of the flow characteristics of a subject's circulating cells.
Current methods for detecting and quantifying various types of cells circulating within a subject's blood stream typically involve extraction of blood from the subject (a patient or an animal) followed by labeling and ex vivo detection. For example, in standard flow cytometry, specific cell populations in a blood sample, drawn from a subject and fluorescently labeled, are passed in single file through a flow stream to be interrogated by a light source (usually a laser). Fluorescence and light scattering signals emitted, or remitted, by the cells in response to the light source can be employed to determine the types and the number of the cells. In another ex vivo conventional technique, known as hemocytometry, cells are counted against a grid while being viewed with a microscope to determine the types of the cells and their numbers.
Such ex vivo techniques, however, suffer from a number of shortcomings. For example, each measurement provides only a single time sample. Consequently, it is difficult to use these techniques to obtain a valid temporal population profile for a cell type of interest that varies unpredictably or rapidly with time, Further, these techniques can suffer from a significant time delay between sample collection and analysis, leading to potential measurement inaccuracies.
Some in vivo techniques for detection of static and circulating fluorescently labeled cells are also known. However, these techniques typically show difficulty, or simply fail, in tracking cells flowing at a high velocity, especially in the arterial circulation, even when they capture images at video rates. In addition, employing these techniques for extracting quantitative information about the number and flow characteristics of a specific cell population can be very tedious.
Hence, there is a need for enhanced methods and apparatus for performing in vivo flow cytometry.